CSI And Firearms

Richard Saferstein (Former Chief Forensic Scientist) gives expert video advice on: How does the study of firearms help in CSI?; How does a gun work?; How are firearms and ammunition collected in CSI? and more...

How does the study of firearms help in CSI?

There are a variety of things that a firearms investigator looks for. He or she will look for, for example, markings on bullets, or the markings on cartridge cases. Also what will be examined perhaps will be residues, or powder residues that are left on a target that has been hit by a bullet, or residues that collect on the hands of a firer, or perhaps a serial number that has been sawed or erased off from a weapon, and hopefully can be restored. So these are all kind of typical examples of the work that a firearms examiner carry out on a daily basis.

How does a gun work?

What happens is you pull a trigger. It releases a firing pin, which in turn impacts with a primer cap. Contained within the primer cap is a small explosive, which in turn sets off a larger explosive, which is mainly due to smokeless powder. That will send the bullet on its way through the barrel of a weapon.

How are firearms and ammunition collected in CSI?

What's important with respect to the forensic characteristics of firearm evidence are the markings that are left, whether it be on a bullet or a cartridge casing, as the weapon, or as the bullet traverses through the firearm. You don't want to disturb those markings. You want to be careful with the way that you handle a collected or fired bullet at a scene, or a cartridge casing at a scene. You do not want to disturb the markings on those items. Likewise, you don't want to disturb the markings within the barrel of the weapon. You don't want to, for example, pick up a weapon with a pencil by inserting it in the barrel because that will disturb the markings, and those are things that are "no-nos" at crime scenes. An important point to be made here is to preserve the markings on the bullet, preserve the markings on the cartridge casings, and preserve the markings within the barrel of the weapon.

What is rifling?

There are a variety of rifling techniques that are used by weapon manufacturers. The purpose of the rifling is to impart into the inner wall of the barrel lands and grooves, upon which the bullet will traverse. It's sort of like a track, as it travels through the barrel it gives the bullet a spin which enables one to control the trajectory of the bullet. It's this spinning action which indeed gives us that control. So rifling is the manufacturing process that imparts lands and grooves to the inner walls of a barrel.

How does studying the rifling pattern of a gun help in CSI?

When a manufacturer of a weapon creates rifling, or lands and grooves within the inner morals of a barrel, there are a variety of techniques that are used and the variety of lands and grooves that can be used. So what is looked for are, a) the number of lands and grooves, b) the direction of twist and c) the angle of twist. Knowing these perimeters will help a forensic investigator perhaps identify the make and model of a weapon that was used in a crime.

How can two shotgun shells be compared in CSI?

Unlike a rifled weapon, the inner walls of a shotgun are smooth and therefore do not contribute markings to the pellets that travel through the barrel. It is the cartridge casing that may be deposited at the crime scene that is left with unique markings, whether it be breach-block markings or firing pin markings, and these types of markings are what the firearm examiner will study and compare when it comes to the identification and individualization of a shotgun.

How are bullets and firearms compared in CSI?

That's easy. There's only one major testing method, and that is the comparative microscope. Physically taking a bullet and comparing it to another bullet, and looking to see whether the stria, or rifling markings, match up underneath the comparison microscope. This is an endeavor that can only be undertaken by the trained eyes of a firearm examiner.

Why is gun powder residue studied in CSI?

When a bullet penetrates a target, if the weapon was close to the target (generally within three feet), residual material from the gun powder may be deposited upon the target. By studying the pattern of the deposition of that material around the bullet hole, a forensic scientist may be able to ascertain the distance the weapon was held from the target.

Why is primer residue on a shooter's hands studied in CSI?

When it comes to some weapon, when they're fired, a primer residue may be deposited on the hands of the firer. This primer residue consists mainly of trace elements. With the presence of these trace elements, a forensic scientist may be able to ascertain whether an individual fired a weapon or was in close proximity to a fired weapon.

How is primer residue tested for in CSI?

The major ingredients of primer residue are barium, anemone, and lead. Forensic scientists use sophisticated instrumentation to determine the elemental composition of materials that may contain barium, anemone and lead.